Thread: getting input from keyboard, passing to execvp

Hi, I'm trying to create a simple shell program that reads in a command from the keyboard and passes it to execvp to execute. I'm working in a Linux environment. I'm splitting the input line into a command (the first token) and arguments (the remaining tokens). For example, ls -l has ls as command and -l as the sole argument. The problem is that execvp does not recognize the first argument. If I input "ls -l", it executes just "ls" and drops the "-l." If I input "ls -l -a", then it executes "ls -a." If I input "ls," it doesn't execute anything. Any thoughts? Thanks! Here is my code:

Code:

#include <unistd.h>
#include <iostream>
#include </usr/include/errno.h>
#include </usr/include/sys/errno.h>
#include <string>

using namespace std;

int main(int argc, char *argv[])
{
        // issue prompt
        cout << "> ";

        // get input from user
        string inputStr;
        char inputStrArr[128];
        char * argArr[128];
        char separators[]   = " ,\t\n";
        char *token, *command;
        int numArgs = -1;
        int k;
        int i;

        getline(cin, inputStr);
        for(i=0; i<inputStr.length(); i++)
                inputStrArr[i] = inputStr[i];
	inputStrArr[i] = '\0';
	token = strtok(inputStrArr, separators);
	command = token;

        while( token != NULL )
        {
                token = strtok( NULL, separators );

                argArr[++numArgs] = token;
        }

        argArr[++numArgs] = '\0';
 
        // fork
        int childpid, flag;

        if ((childpid = fork()) == 0) // child code
        {
                cout << "Process ID: child: " << getpid();
                flag = execvp(command, argArr);

                if (flag < 0)
                {
                        perror("execvp failed");
                }
        }
        else if (childpid > 0) // parent code
    {
        cout << "Process ID: " << getpid();
    }
    else // fork unsuccessful
    {
        cout << "\nError: fork failed\n";
    }

return 0;
}

Thanks Salem, I missed that one!

Now I have another question about comparing the value in a char pointer.
I need to see if the last token on the line is "&" -- which indicates a background process. If so, it needs to be excluded from the argument array and a bool flag set. The problem is I don't know how to compare the value in this last token with "&". Here is my code:

Code:

#include <unistd.h>
#include <iostream>
#include </usr/include/errno.h>
#include </usr/include/sys/errno.h>
#include <string>

using namespace std;

int main(int argc, char *argv[])
{
        cout << "program has started\n";

        // issue prompt
        cout << "> ";

        // get input from user
        bool background = false;
        string inputStr;
        string commandStr;
        char inputStrArr[128];
        char * argArr[128];
        char separators[]   = " ,\t\n";
        char *token, *command;
        int numArgs = -1;
        int k;
        int i;

        getline(cin, inputStr);
        while (inputStr.length() == 0)
        {
                cout << "> ";
                getline(cin, inputStr);
        }

        for(i=0; i<inputStr.length(); i++)
                inputStrArr[i] = inputStr[i];
        inputStrArr[i] = '\0';
                                                                   
	token = strtok( inputStrArr, separators );
        command = token;

        for (i=0; token[i] != '\0'; i++)
        {
                commandStr += token[i];

        }

        argArr[++numArgs] = token;

        while( token != NULL )
        {
                token = strtok( NULL, separators );
                argArr[++numArgs] = token;
        }

        if (argArr[numArgs] == '&')
        {
                background = true;
                argArr[numArgs] = NULL;
        }
        else
                argArr[++numArgs] = NULL;

    // is command built-in?
        if(commandStr == "quit")
        {
                cout << "============= quit called\n";
                return 0;
        }
        else
        {
	    // execute external command
	        int childpid, flag;
	        childpid = fork();
	        cout << "forking\n";
	        if (childpid == 0) // child code
	        {
	                cout << "Process ID: child: " << getpid();
	                flag = execvp(command, argArr);
	
	                if (flag < 0)
	                {
	                        perror("execvp failed");
	                }
	        }
	        else if (childpid > 0) // parent code
	        {
	                cout << "Process ID: " << getpid();
	        }
	        else // fork unsuccessful
	        {
	                cout << "\nError: fork failed\n";
	        }
	} // end else
return 0;
}

Thanks again. I've got my shell working well at this point. However, I would like to create a class and split the main pieces of code into functions. The problem I have is with the get_command function. It takes the command from the user and does everything as before. The client then should be able to access the command and arguments. However, I must not be passing the argments to get_command properly, because I'm getting a segmentation error when the client tries to access "command" and "args." Here is the function delaration:
void get_command(char* command, char* args[]);
and the client code:

Code:

        char * argArr[128];
        char * command;
        shell1.get_command(command, argArr);

I tried passing the arguments as pointer references:
void get_command(char*& command, char*& args[]);
but no luck there, either. Then the program won't even compile. Any ideas? Thanks!